An optical encoder, generally, detects a rotation angle of an rotation axis, a rotation speed and a position and a speed of a linearly-moving object by emitting parallel beams of light from a light source to an optical scale having light transmission portions and light nontransparent portions which are alternately arranged, converting the light into a modulation signal and further converting the modulation signal into an electrical signal with a light detecting element.
An exemplary optical encoder of the background art having a purpose of lower cost and higher precision is disclosed in, for example, Patent Document 1. In Patent Document 1, it is described that a conventional technique has a problem of high cost since an optical scale is manufactured through pattern formation “by chromium evaporation of a glass plate and etching of the chromium layer to make a transparent portion and an opaque portion”, and in order to solve this problem, a light shielding portion (a light non-transparent portion) is provided by additionally forming an inclined portion between light transmission portions by e.g., resin molding so that an incident angle of an incident ray of light is not smaller than a critical angle and the light shielding portions and the light transmission portions are alternately arranged to make an optical scale like a slit row made by chromium evaporation, which achieves lower cost through resin molding. In description on FIG. 2 showing one preferred embodiment of Patent Document 1, it is discussed that “it is set that the critical angle is 45 degrees or less, the angle made by extensions of inclined surfaces such as 10a and 10b which constitute one convex portion of an optical grating is 90 degrees, an incident angle of light entering the inclined surfaces 10a and 10b is 45 degrees and an incident angle of light entering flat surfaces such as 9a and 9b is 0 degree”, and “the incident light on the inclined surface 10a which has an incident angle of 45 degrees is totally reflected by 90 degrees to enter the other inclined surface 10b at the angle of 45 degrees and then is totally reflected thereon by 90 degrees, going back to the incident side.”
Among other background-art cases are Patent Documents 2 and 3.
Patent Document 2, for example, shows an optical scale in which light transmission portions and light non-transparent portions each of which is constituted of inclined surfaces which are set so that an incident angle of an incident light is not smaller than a critical angle are alternately arranged on a surface of a transparent member formed of polycarbonate, and it is discussed in the document that since a range of incident angle of total reflection is wide when a polycarbonate is used, even if the incident light obliquely enters the optical scale, there is high probability that the light should be totally reflected to go back to the former side and therefore it is unlikely to cause stray light.
In Patent Document 3, it is discussed that an optical path changing function has a shape with projections or depressions which are sufficiently smaller as compared with the thickness of a movable code plate and at least one shape with projections or depressions is formed in at least one of the aforementioned regions, and providing a plurality of structures with projections or depressions suppress the thickness of the structures with projections or depressions.
Patent Document 1: Japanese Patent Application Laid Open Gazette No. 60-140119 (pp. 1 to 2, FIGS. 1 and 2)
Patent Document 2: Japanese Patent Application Laid Open Gazette No. 62-5131 (p. 1 to 3, FIGS. 1 to 3, and 5)
Patent Document 3: Japanese Patent Application Laid Open Gazette No. 11-287671 (page 4, FIGS. 1 to 3)